Very small homes and apartments have recently become very popular. When space is limited, furniture is often used that combines several functions in one piece of furniture. Combining a bed and desk into a single peace has become a popular idea. It is well known to provide desk/bed combinations in which the bed is positioned over the desk. This combination put the bed at a height that put the user at great risk if the user fell from bed. Young children are a greater risk than adults of falling out of bed. Furthermore, parents like to tuck their children in bed which is difficult to do when the bed is elevated above a desk.
The most likely reason the bed has been put above the desk in the aforementioned prior desk/bed combinations is that in all of the identified prior arrangements, the separation between the bed and the desk has been a horizontal plane. If a horizontal plane separates the desk and bed, then space constraints would make it impractical to put the desk above the bed. If the desk were put above the bed, then the vertical space required for a person in bed would cause the desk to be impractically high off of the floor.